1. Field of the Invention
The present invention pertains to optics, and in particular, to optics in microlithography.
2. Background Art
Photolithography (also called microlithography) is a semiconductor fabrication technology. Photolithography uses ultraviolet (UV) or visible light to generate fine patterns in a semiconductor device design. Many types of semiconductor devices, such as, diodes, transistors, and integrated circuits, can be fabricated using photolithographic techniques. Exposure systems or tools are used to carry out photolithographic techniques, such as etching, in semiconductor fabrication. An exposure system can include a light source, reticle, optical reduction system, and a wafer alignment stage. An image of a semiconductor pattern is printed or fabricated on the reticle (also called a mask). A light source illuminates the reticle to generate an image of the particular reticle pattern. An optical reduction system is used to pass a high-quality image of the reticle pattern to a wafer.
Integrated circuit designs are becoming increasingly complex. The number of components and integration density of components in layouts is increasing. Demand for an ever-decreasing minimum feature size is high. The minimum feature size (also called line width) refers to the smallest dimension of a semiconductor feature that can be fabricated within acceptable tolerances. As a result, it is increasingly important that photolithographic systems and techniques provide a higher resolution.
One approach to improve resolution is to shorten the wavelength of light used in fabrication. Increasing the numerical aperture (NA) of the optical reduction system also improves resolution. Indeed, commercial exposure systems have been developed with decreasing wavelengths of light and increasing NA. For example, UV exposure systems are available from ASML, Inc. which have light sources operating at a wavelength of 248 nanometer (nm) with an associated NA of 0.5 or 0.6, at a wavelength of 193 nm with an associated NA of 0.5 or 0.6, and at a wavelength of 157 nm with an associated NA of 0.75.
In UV microlithography, it is important to maintain a high light transmissivity through an optical reduction system with little or no loss. Exposure time and the overall semiconductor fabrication time depends upon the intensity or magnitude of light output onto the wafer. An optical reduction system must also output a sharp focused image of a mask onto a wafer. This ensures that the fine detail needed for semiconductor fabrication is preserved.
As the exposure wavelength decreases, an optical reduction system must include optical components, such as, lenses, which are made of a material which is transparent even at low UV wavelengths such as 193 nm and 157 nm. Examples of such optical materials include calcium fluoride (CaF2) and barium fluoride (BaF2). These optical materials, however, have a relatively high degree of intrinsic birefringence (also called spatial-dispersion-induced birefringence). This high intrinsic birefringence is very direction dependent. As a result, the optical characteristics of the optical material (such as transmissivity and refraction) vary unevenly across a beam incident on the optical material. In other words, because of the directional dependence of the intrinsic birefringence, some parts of a beam spot may be sped up or slowed down relative to other parts of the beam spot depending the polarization of light at the different parts of the beam spot. In demanding applications like microlithography, such intrinsic birefringence is undesirable as it can blur or reduce the sharpness of an image, or cause loss of light through an optical reduction system.
One approach to correcting intrinsic birefringence is to use a single pair of optical elements rotated relative to one another. For a single pair of lenses having a <100> crystal orientation, the optical axis of the crystal structure of one lens in the pair is rotated at an angle relative to the optical axis of the crystal structure of other lens. Such correction of intrinsic birefringence is limited, especially in high-quality applications like photolithography.
What is needed is an even superior approach for correcting intrinsic birefringence than a pair of optical elements. This is especially needed in optical reduction systems used in low UV microlithography applications.